"DirectX 10 is probably the most important revolution in games development, at least since the introduction of the programmable shader in DirectX 8.0. Because of the way that Microsoft has designed the new driver model, DirectX 10 will only be available for Windows Vista users and there will not be a version released for Windows XP. Along with DirectX 10, Windows Vista will come with DirectX 9.0Ex - this is because pre-DirectX 10 hardware will not work under the new API due to the complete overhaul."